Crystals, vibrational analysis

The repeat distance along the chain axis (0.468 nm) is significantly less than that calculated for a planar zigzag stmcture. Therefore, the polymer must be in some other conformation (65—67). Based on k and Raman studies of PVDC single crystals and normal vibration analysis, the best conformation appears to be where the skeletal angle, is 120°, and the torsional of opposite sign) is 32.5°. This conformation is in... 

The term plastic crystal is not used if the rotation of the particles is hindered, i.e. if the molecules or ions perform rotational vibrations (librations) about their centers of gravity with large amplitudes this may include the occurrence of several preferred orientations. Instead, such crystals are said to have orientational disorder. Such crystals are annoying during crystal structure analysis by X-ray diffraction because the atoms can hardly be located. This situation is frequent among ions like BF4, PFg or N(CH3)J. To circumvent difficulties during structure determination, experienced chemists avoid such ions and prefer heavier, less symmetrical or more bulky ions. 

Strahle and co-workers971 were the first to report the synthesis of the dichloroni-tronium ion [Eq. (4.226)]. According to X-ray crystal structure analysis of the hexachloroantimonate salt, which shows a remarkable stability up to 145°C, the cation is almost planar (maximum deviation from the plane is 0.038 A) and has Cs symmetry. The N—O bond distance is 1.31 A, that is, lies between the length of a NO single bond (1.151 A) and a NO double bond (1.47 A). This is indicative of a decreased Tt-bond contribution, which is also reflected in the IR spectrum (stretching vibration at 1650 cm 1 as compared to 1827 cm-1 of phosgene). 

The structural interpretation of dielectric relaxation is a difficult problem in statistical thermodynamics. It can for many materials be approached by considering dipoles of molecular size whose orientation or magnitude fluctuates spontaneously, in thermal motion. The dielectric constant of the material as a whole is arrived at by way of these fluctuations but the theory is very difficult because of the electrostatic interaction between dipoles. In some ionic crystals the analysis in terms of dipoles is less fruitful than an analysis in terms of thermal vibrations. This also is a theoretically difficult task forming part of lattice dynamics. In still other materials relaxation is due to electrical conduction over paths of limited length. Here dielectric relaxation borders on semiconductor physics. 

Tel. 617-495-4018, fax 617-495-1792, e-mail karplus huchel.bitnet Molecular dynamics package using Chemistry at Harvard Macromolecular Mechanics force field. Extensive scripting language for molecular mechanics, simulations, solvation, electrostatics, crystal packing, vibrational analysis, free energy perturbation (FEP) calculations, quantum mechanics/molecular mechanics calculations, stochastic dynamics, and graphing data. 

Besides vibrational and NMR spectra, a crystal structure analysis is reported for [CH3SBr2 ][AsF6 ]. Apart from a Br- F distance of 2.876 A the trigonal p3Tamidal CH3SBr2 ion (S-Br 2.204, 2.259 A S-C 1.851 A) has no significant intermolecular secondary bonding contacts (313). 

Although the effective Hamiltonian can be justified by orthogonal projection or by the respective perturbation theory in the OOA it is just posmlated. Analysis of the background theory reveals its physical meaning. The OOA is based on a shift transformation to a minimum point of APES for an elementary cell in the respective low-symmetry mean field of all other cells of the crystal. Vibrational motion of ligands is averaged out [36]. 

The traditional ways of evaluating the IHB characteristics are to assess the vibrational frequencies or intensities of the OH stretching or torsion in the IR spectra and the chemical shifts of the hydroxyl protons in the NMR spectra which are found to be nicely correlated ". Crystal-structure analysis also provides essential information in this field. Bilton and coworkers carried out a systematic survey of the internally hydrogen-bonded frames in the 200,000 structures of the CSD and gave a general overview of the IHB in the solid state. 

In this context, it is nevertheless important to know if )3 turns in proteins make specific contributions to the spectra. The only protein for which such a vibrational analysis has been done is insulin (Bandekar and Krimm, 1980). This protein is a particularly suitable one for such a study, since its structure has been solved (Blundell et al., 1972), it is relatively small, with only four )3 turns, and Raman spectra of single crystals have been reported (Yu et al., 1974). The normal-mode calculations (Bandekar and Krimm, 1980) permit a correlation of previously unassignable bands in the Raman spectrum with turns in the structure, as well as showing that some of the computed )3-turn frequencies lie in spectral regions previously associated exclusively with a-helix modes. These results thus emphasize our previous remarks that caution must be exercised in proposing unique assignments of bands to a-helix and /3-sheet structures in proteins. 

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